Esters are usually prepared by the reaction of alcohols or phenols with acids in the presence of a mineral acid catalyst. ##STR1## The reaction comes to equilibrium with appreciable amounts of the starting materials remaining. The equilibrium can be shifted to the right using a large excess of the alcohol. The water can be removed from the reaction by azeotropic distillation with a suitable solvent, such as benzene, thereby driving the reaction to completion.
There are a number of alternative methods for preparing esters, and each is useful under certain circumstances. The more useful of these methods are indicated below. ##STR2## The reaction of an acid with diazomethane is hazardous except on a very small scale. Ketenes react with alcohols to give esters, but the reaction is of limited usefulness because of the limited availability of ketenes. The reaction of an acid chloride with an alcohol is usually used when it is desired to convert a valuable alcohol to an ester, usually the acetate. Most often the reaction is carried out in pyridine, which forms a salt with the HCl generated and prevents the solution from becoming acidic. An anhydride can be used in place of an acid chloride in a similar way. Transesterification is the reaction of an ester with an alcohol to yield a different ester. Transesterification is catalyzed by acids as well as bases.
In general, aliphatic and aromatic acids can be converted to esters by all of the methods referred to above. There are, however, limitations on the alcohols that are satisfactory in these reactions. Primary and secondary alcohols can generally be used in esterification of all kinds. Tertiary alcohols cannot be esterified in the presence of acids because they are easily converted to carbonium ions, which then undergo elimination or other reactions. Esterification of tertiary alcohols with acid chlorides can be effected under basic conditions.
Synthesis gas may be defined as any of several gaseous mixtures used for synthesizing a wide range of compounds, both organic and inorganic. Such mixtures result from reacting carbon-rich substances with steam (steam reforming) or steam and oxygen (partial oxidation). These mixtures contain chiefly carbon monoxide and hydrogen, and usually low percentages of carbon dioxide and nitrogen (less than 2%). The organic source materials may be natural gas, methane, naphtha, heavy petroleum oils or coke. The reactions are usually nickel-catalyzed steam-cracking (reforming) of methane or natural gas (CH.sub.4 +H.sub.2 O.fwdarw.CO+3H.sub.2); partial oxidation of methane, naphtha, or heavy oils; and the water-gas reaction with coke (C+H.sub.2 O.fwdarw.CO+H.sub.2).
Processes are known for converting synthesis gas to alcohols, aldehydes, acrylic acid, etc. These processes usually involve contacting synthesis gas with a transition-metal catalyst at elevated temperatures and pressures. For example, U.S. Pat. No. 4,298,354 discloses a process for converting synthesis gas to alcohols using an oxide-complex catalyst containing copper, thorium, an alkali metal and at least one other metal selected from the group consisting of Ca, Mo, Rh, Mn, Pt, Ce, Cr, Zn, Al, Ti, La, V, U, Ru, Re and Pd. U.S. Pat. No. 4,377,643 discloses the production of alkanes and oxygenated hydrocarbons, particularly alcohols, friom synthesis gas using a catalytic complex containing ruthenium, copper, an alkali metal and a promoter selected from the group consisting of Rh, Ir, Pd, and Pt.